1. Field of the Invention
The present invention relates to an angle detection method with a resolver which detects a rotational angle of a rotator such as a motor by a signal obtained from a one phase excitation/two phase output resolver.
2. Description of the Related Art
A method which uses a one phase excitation/two faze output resolver is known as a method for detecting a rotational angle of a rotor of a rotating machine or the like. The one phase excitation/two phase output resolver receives an input of an excitation signal that is an alternating current signal and sends two phase signals (two phase sine wave modulation signals) in which amplitude modulation is performed by a cosine value and a sine value of a rotational angle using a coil excited by the excitation signal. Hereinafter, the respective phase signals are referred to as a first signal and a second signal respectively.
In the angle detection with the one phase excitation/two phase output resolver, the first signal and the second signal are detected with codes, resolver-to-digital conversion (referred to as “RD conversion” later) is performed by using a four quadrant arc tangent (a tan 2) in which the first signal and the second signal are used as inputs.
Here, the four quadrant arc tangent will be described. The usual arc tangent uses an amplitude ratio; and therefore, calculated results are in a range of −90 to +90 degrees. However, in the four quadrant arc tangent, not only the amplitude ratio but also codes of a sine signal (referred to as “sin signal” later) and a cosine signal (referred to as “cos signal” later) are taken into account; and accordingly, the calculated results can be obtained in a range of −180 to +180 degrees. For example, in the case where the first signal is the cos signal and the second signal is the sin signal, the four quadrant arc tangent is divided into the following cases: a first quadrant if the cos signal and the sin signal are both positive; a second quadrant if the cos signal is negative and the sin signal is positive; a third quadrant if the cos signal is negative and the sin signal is negative; and a fourth quadrant if the cos signal is positive and the sin signal is negative. Accordingly, the rotational angle can be obtained in the range of −180 to +180 degrees. The rotational angle can be obtained by the RD conversion using the four quadrant arc tangent.
The excitation signal of such a one phase excitation/two phase output resolver is the alternating current signal and the first signal and the second signal are alternating current signals synchronized with the excitation signal. In the case where the rotational angle is detected on the basis of the first signal and the second signal, the amplitude ratio of the first signal to the second signal is used; and therefore, the amplitude accuracy of the first signal and the second signal influences the detection accuracy of the rotational angle. The first signal and the second signal that are alternating current signals change their amplitude by detection time; and therefore, in the conventional angle detection method with the resolver, the first signal and the second signal are simultaneously detected in synchronization with the excitation signal. The reason is that, for example, if the second signal is detected after the first signal is detected, the second signal corresponding to the time at which the first signal is detected cannot be detected; and, as a result, the accuracy of the amplitude ratio of the first signal to the second signal cannot be maintained and position detection accuracy is also degraded.
The angle detection method with the resolver which simultaneously detects the first signal and the second signal of two phase outputs can maintain position detection accuracy by accurately obtaining the amplitude ratio of the first signal to the second signal; however, in order to simultaneously detect two phase output signals, hardware such as a microcomputer and a sample and hold circuit, both having a function capable of simultaneously detecting a plurality of signals, needs to be used. Such a microcomputer which detects the signals by providing a time difference is more inexpensive than the microcomputer which simultaneously detects a plurality of signals. However, problems exist in that if hardware such as the sample and hold circuit is added to such a microcomputer in order to simultaneously detect the first signal and the second signal, it causes cost-up by the hardware and the size of the entire circuit is increased by the size of the hardware.
In order to solve this problem, for example, in a conventional angle detection method disclosed in Patent Document 1, a signal processing method of a resolver which sends a rotational angle on the basis of two phase sine wave modulation signals includes: with respect to two phase signals sent from the resolver, a process of sampling signals of respective phases of the two phase signals respectively at a predetermined timing while sandwiching outbreak time of the peak amplitude of the two phase signals while a rotator is substantially stopped; and a process of calculating an angle on the basis of the respective sampled signals while performing correction of errors due to deviation of the timing of sampling between the respective phase signals.
In such a conventional angle detection method, it is supposed that the respective timings of sampling continuously performed two times are deviated by equal times from the peak respectively while sandwiching the peak; and therefore, the amplitude ratio of the first signal to the second signal can be accurately detected without simultaneously detecting the first signal and the second signal of the two phase outputs. As a result, an accurate rotational angle can be obtained without using hardware such as the microcomputer and the sample and hold circuit, both having the function capable of simultaneously detecting the plurality of signals.